The present invention relates to a new method of making high temperature resistant FeCrAl-steel strip from ferritic (FeCr) stainless steel by using physical deposition technology which usually is called "physical vapor deposition" (PVD)-technique. Steel strips rich in aluminum are used in applications where there is high temperatures as well as a quick change in temperature. One example is thin foils for metallic catalytic converters where aluminum is used to form oxides that protect the steel surface from corrosive attacks. One great advantage of the invention is the increased flexibility in varying the analysis of said steel, primarily in regard to the aluminum content.
It is prior art to use FeCrAl-alloys for heat resistant applications such as purification of automobile gases by using metallic catalytic converters. Aluminum is added to the steel to form aluminum oxide which is one of the most stable oxides having a low speed of oxygen diffusion. Therefore, the aluminum oxide provides a good protection against corrosion at high temperatures in an oxidation atmosphere. Those alloys are difficult to manufacture by conventional steel-making from scrap-based metallurgy via casting, hot rolling and cold rolling. Inevitably both macroscopic and microscopic inclusions will be formed, mainly aluminum-based which will complicate their manufacture and contribute to less efficiency primarily when cold rolling the material down to its final dimension. Intermetallic phases will be formed at 450-500.degree. C. and those precipitations will reduce the toughness of the material and make the steel brittle. This means that the heat treatment steps involved in the manufacture must be carried out very carefully, i.e., the material must be quenched below that area where intermetallic phases are being formed. It is established that conventional manufacturing technology puts a limitation to the aluminum content at maximum of 6% in order to minimize such process problems.
There are other methods available for the making of this type of material. One method includes rolling a ferritic stainless steel jointly with an aluminum foil (U.S. Pat. Nos. 5,366,139 and 5,447,699). A ferritic chromium steel is rolled to suitable band thickness after which said band is rolled jointly with a surrounding aluminum foil to a final thickness. Such cold rolled composite material consists of three layer that is heat treated to obtain a homogenous material with aluminum evenly distributed in the ferritic matrix.
Another variant includes dipping the steel band into a bath of liquid aluminum or an aluminum alloy (U.S. Pat. Nos. 3,907,611, 3,394,659 and 4,079,157). This method, however, is complex and it is difficult to control the process parameters. This causes large manufacturing costs which are not commercially feasible.